The present invention relates to high voltage semiconductor switching devices. The invention more particularly, although not exclusively, relates to switching device for switching converter applications.
Prior art switching converter circuits are shown in FIGS. 1 and 2. FIG. 1 shows an isolated constant voltage output switching converter while FIG. 2 shows a non-isolated switching converter having constant current output for driving LED lighting.
In both FIGS. 1 and 2, the selection of the power switching device (115 and 215 respectively) is a key decision. Bipolar transistor and MOSFET are typical candidates for power switching device. Bipolar transistors are much less costly than the MOSFET at the same power rating. However, MOSFETs are normally more preferred, especially at high power levels, due to the following reasons:                a. Bipolar transistors require continuous base current to keep them in the turn on state while MOSFETs only require the charge up of the gate capacitance to turn them on;        b. The current gain for power bipolar transistors with high breakdown voltage (say, 600V or more) is usually not high (say, at around 10 to 20, or even less than 10). This renders the power for driving the base substantial, especially when the power converter delivers high power to its output. The efficiency of the switching converter circuit will then be degraded.        
By using bipolar transistors in a Darlington configuration, effective current gain becomes product of individual transistor current gain. Hence, effective current gain of a few hundred can be obtained easily and the power loss due to base driving can be reduced to comparable with the gate driving for MOSFET counterpart at the same power level. However, commercially available Darlington transistors are normally being offered in three pin package in which B is the first base and E is the last emitter as shown in FIG. 3. It is easy to turn on by a small base current but the turn off is very slow due to base relaxation at the inner base (base pin for transistors 302 and 304 in FIG. 3). Therefore, it is not suitable for switching converter application as slow switching transition generates substantial amount of heat at the switching device, as well as degradation of efficiency.
IGBT is a hybrid power switching device that attempts to combine the advantage of MOSFET and bipolar transistor. For IGBT, the power for the gate driving is small as it is the charge up of the control gate capacitance similar to the case of MOSFET while the chip size is similar to bipolar transistor for the same rated current due to the conduction mode is bipolar action. However, due to the actual bipolar base terminal is inside the device, it suffers the same problem of slow switching off as the three terminal Darlington bipolar transistor.
As of today, MOSFET is still the most widely used choice for power switching device.